There exists a need to determine the lower-bound density of a substance within a volume of interest from a number of different projections of a body containing the substance. In this context, a "projection" is obtained by scanning the volume of interest with penetrating radiation across several predetermined areas so that each scan permits the mass of the substance encountered by the scanning medium to be determined. In this case, each projection gives the total mass of the substance encountered along the respective line of sight of the projection.
Examples where such techniques may be employed include medicine where, for example, tumours may have a different density from that of the surrounding healthy tissue. Thus, it is possible to discriminate between the tumour and the healthy tissue by determining the lower-bound density of all tissue within a volume containing the suspected tumour. The terms "mass" and "density" are used here as an example of any property of the volume of interest which is measured by the scanning medium. Thus, explosives containing high proportions of particular elements or molecules may be discriminated from other materials by determining the lower-bound density of the particular elements or molecules of all objects within a suspected volume.
Prior art systems for discriminating between different bodies within a given volume are typically based on a tomographic analysis of images derived from a series of many projections or views. Such an approach requires the use of complex algorithms which is time-consuming and the further need to view the volume of interest from many different angles adds significantly to the cost, the duration of the test and the radiation dosages required.
In U.S. Pat. No. 4,941,162 (Vartsky et al.) there is described a method and system for the detection of a nitrogenous explosive material in an object. The method described by Vartsky et al. involves scanning the object with a .gamma.-ray beam derived from a suitable source of radiation disposed on one side of the object, and detecting resonant attenuations of the incident photon flux produced by that beam on an array of detectors having a nitrogen rich detecting medium. In such a method, an explosive material containing a large volume of nitrogenous material produces a high reading on the detector, whilst non-nitrogenous materials produce a low reading or no reading at all. As is well known, for penetrating electromagnetic radiation such as .gamma.-rays, ##EQU1## where: D=distance of detector from source
x=line of sight PA0 I.sub.o =incident flux PA0 I.sub.d =detected flux PA0 A=constant of proportionality PA0 K=absorption constant PA0 .rho..sub.a =density of absorbing atom PA0 m.sub.a =cumulative mass along line of sight of absorbing atom.
For resonant attenuation, the density of all but the resonating atom may be neglected and the above logarithm is proportional to the integral along the line of sight of the resonating atom's density, i.e. its mass.
The present invention finds application, inter alia, in analysing the image data produced by such a method in order to discriminate between nitrogenous explosives and other material, based on the nitrogen density.
There exists a continuing need in airport security systems to provide fail-safe systems for identifying explosives within passengers' luggage. Such methods and systems are subject to two stringent requirements. On the one hand, they must be sufficiently sensitive that they successfully identify explosives which, if not otherwise located, would constitute a security risk. At the same time, they must not be so sensitive that they give false alarms since even a very small failure rate, giving rise to a false alarm, is unacceptable in airport security systems. This will be understood, more clearly, when it is considered that for each suitcase containing an explosive, there are some 100.times.10.sup.6 which are perfectly safe. Thus, a failure rate of only one in a thousand will yield 100,000 false alarms to each true alarm, which is clearly quite unacceptable.